Magnetic shields are generally used for preventing influence of magnetic field-generating sources such as magnetized articles on other articles or electric circuits. A commonly used class of magnetic shields are sheet metals having high magnetic permeability and hence, high shielding properties although the sheet metals have only limited versatility in view of nature and cost.
Another class of magnetic shields are powder materials which can be advantageously applied in various ways. For example, magnetic powder is dispersed in organic binders to form coating compositions which are either directly applicable to sites to be shielded against magnetism or coated onto suitable flexible supports to form shielding plates.
A number of high magnetic permeability powders were proposed as magnetic shield materials.
Japanese Patent Application Kokai No. 201493/1984, for example, discloses a magnetic shield coating composition comprising flat particles obtained by finely dividing a soft magnetic amorphous alloy and a polymeric binder. Japanese Patent Application Kokai No. 59268/1983 discloses a magnetic shield coating composition comprising flat particles of a high magnetic permeability alloy dispersed in a polymeric binder. Japanese U.M. Publication No. 50495/1983 discloses to coat Sendust alloy flakes to form a magnetic shield film. Japanese Patent Publication No. 58631/1987 discloses a magnetic shield coating composition comprising flat, irregularly shaped particles dispersed in a polymeric binder, the particles being of Fe-Ni alloy, Fe-Ni-Co alloy, Fe-Si-Al alloy, and Fi-Ni-Mo alloy, which are commercially available as Permalloy, Molybdenum Permalloy, and Sendust alloy. Japanese Patent Publication No. 39966/1988 also discloses Permalloy magnetic shield films. Further, Japanese Patent Application Kokai No. 223627/1989 discloses magnetic shield protective films which are prepared by coating flat magnetic iron powder consisting of iron and either one of 0.5 to 20% by weight of Cr, 0.5 to 9% by weight (or 1 to 16.5 atom%) of Si, and 0.5 to 15% by weight of Al.
Flat alloy particles are often used in these magnetic shield films and compositions for the following reason. In coating such compositions, flat alloy particles are oriented such that their major surface is parallel to the coating surface. This means that the direction of flatness of particles coincides with the direction of magnetic shields on use, allowing the magnetic shields to take full advantage of the high magnetic permeability of the particles themselves due to the reduced diamagnetic field attributable to the flat geometry. Good magnetic shielding properties are provided since any loss of magnetic properties in a direction parallel to the coating surface by the influence of diamagnetic field is avoided.
Nevertheless, the conventional well-known alloy powders for magnetic shields have several problems.
Among Fe-Si-Al alloys, one having the composition of 9.6 wt % Si, 5.4 wt % Al, and the balance of Fe and exhibiting a highest maximum permeability .mu.m is designated Sendust alloy. Sendust alloy suffers from inconvenience of handling because it is less corrosion resistant and in particular, becomes pyrophoric when divided into flat shape because of an increased specific surface area. It is also prone to rusting so that it detracts from magnetic properties and outer appearance. In addition, Sendust alloy has a saturation magnetostriction constant which is less than about 0.3.times.10.sup.-6, but cannot be negative or lower than 0. When it is desired to use Sendust alloy as magnetic shielding material by flattening it into, flat particles, its magnetic properties can be deteriorated by stresses applied during flattening process and use, failing to meet the magnetic shield design requirement.
Permalloy type alloys including Permalloy and Molybdenum Permalloy are flattened through a rolling process rather than cleavage because of their crystal structure and thus require a longer time to flatten, leading to low productivity. The increased time of flattening process induces more stresses in particles, failing to provide high magnetic shielding properties. In addition, the Permalloy type alloys are about 5 to 10 times more expensive than the Sendust alloy.
Iron base amorphous alloys also give rise to problems as found with the Permalloy type alloys since they are flattened through rolling. Moreover, Permalloy type alloys and iron base amorphous alloys have increased magnetostriction and thus detract from magnetic properties not only through stress application during flattening, but also through stress application during milling with binder to form a coating composition. Another drawback of Permalloy type alloys is associated with their softness in that flat particles are liable to deform by stresses induced during milling to form a coating composition, also resulting in a loss of magnetic properties.